Coccolith Sr/Ca as a new indicator of coccolithophorid calcification and growth rate

Abstract

Polyspecific coccolith separates from core top sediments in the eastern equatorial Pacific show variations of ∼15% in Sr/Ca ratios across the equatorial upwelling zone, with Sr/Ca highest at the equator and decreasing off‐axis. These variations cannot be due to changes in the Sr/Ca of seawater, which varies by less than 2% in the surface ocean. Variations in Sr/Ca of coccolith sediments are similar to variations in primary productivity and alkenone‐estimated coccolithophorid growth rates in overlying surface waters and to CaCO3 rain rates measured in sediment traps. Because of these relationships and because calcification rate exerts a strong control on Sr/Ca in abiogenic calcites, we suggest that the observed Sr/Ca variations in coccoliths may be strongly controlled by coccolithophorid growth and calcification rates, although temperature may also influence coccolith Sr/Ca to a lesser degree. Changes in dissolution intensity and coccolith assemblages appear to exert a minor influence, if any, on coccolith Sr/Ca in these sediment core tops. If further work confirms relationships between coccolith Sr/Ca and coccolithophorid productivity, Sr/Ca records of past changes in coccolithophorid productivity may be useful in reconstructing past variations in the rain ratio of organic to carbonate carbon, an important control on deep ocean pH and partitioning of CO2 between the atmosphere and ocean. In addition, coccolith Sr/Ca might provide an independent record of past changes in coccolithophorid growth rates, which in combination with data on the carbon isotopic fractionation in coccolithophorid organic matter may permit more reliable calculations of past dissolved CO2 in the surface ocean.